1. Field of the Invention
This invention relates to improving the anticorrosive properties of an autodeposition coating by a post-bath rinse using an aqueous solution of an alkaline earth metal compound such as calcium nitrate.
2. Discussion of the Related Art
Over the last few decades, various water-based coatings for metallic surfaces have been developed which are commonly referred to in the field as autodeposition coatings. Such coatings utilize an emulsion (latex) or dispersion of a resin capable of forming a protective coating when cured. The coating typically is applied by immersing the metallic surface in a bath containing the resin emulsion or dispersion, acid, and an oxidizing agent to form an adherent coating that is initially wet. The thickness of the coating can be affected, for example, by such factors as total solids, pH and oxidant concentration. Further, the coating thickness is a function of the immersion time. The initial wet coating is sufficiently adherent to remain attached to the surface on which it is formed against the influence of normal gravity and, if desired, can be rinsed before being cured (i.e., converted to a dry, solid and even more adherent coating) by heating. However, a coating produced in this manner does not always provide adequate resistance against corrosion for the metal substrate, as determined, for example, by standard salt spray tests.
The corrosion resistance of certain autodeposited coatings is significantly improved by rinsing the adhered coating, prior to curing, in an aqueous solution containing chromium ions. Appreciable chromium ion concentrations are required to give acceptable coatings. The chromium rinse step is undesirable from an economic and environmental perspective, since chromium compounds are generally both expensive and highly toxic.
The abovedescribed autodeposition coating compositions and coating and rinsing procedures are more fully described in U.S. Pat. Nos. 3,063,877; 3,585,084; 3,592,699; 3,647,567; 3,791,431; 4,030,945; 4,186,226; 3,795,546; 4,636,265; 4,636,264; and 4,800,106, each of which is incorporated herein by reference in its entirety.
From the present state of the art, as above-described, it will be appreciated that there is a need for coating methods capable of producing adherent metal coatings possessing satisfactory corrosion resistance properties without requiring a rinse step where a chromium-containing solution is utilized.
In one embodiment of the invention, a method for improving the anticorrosive properties of a resin autodeposited on a metal substrate is provided, said method comprising
(a) contacting said metal substrate with an autodeposition bath containing said resin in uncured emulsion or dispersion form and an autodeposition activator until a layer of the resin of desired thickness is autodeposited on said metal substrate;
(b) rinsing said metal substrate having the layer of resin autodeposited thereon with a chromium-free aqueous solution containing an anticorrosive effective amount of a water-soluble alkaline earth metal compound; and
(c) curing the layer of resin autodeposited on said metal substrate following rinsing step (b).
Specific preferred and/or illustrative embodiments of the invention are as follows:
The foregoing method wherein the water-soluble alkaline earth metal compound is a calcium compound.
The foregoing method wherein the water-soluble alkaline earth metal compound is a nitrate compound.
The foregoing method wherein the water-soluble alkaline earth metal compound is calcium nitrate.
The foregoing method wherein said resin comprises an epoxy resin.
The foregoing method wherein step (b) is performed at a temperature of from about 20xc2x0 C. to about 100xc2x0 C.
The foregoing method wherein the aqueous solution has a concentration of the water-soluble alkaline earth metal compound of from about 0.1 to about 5 percent by weight.
In another embodiment, this invention provides a method for improving the anticorrosive properties of a resin comprising an epoxy resin autodeposited on a metal substrate, said method comprising
(a) contacting said metal substrate with an autodeposition bath containing said resin in emulsion form and an autodeposition activator until a layer of the resin of desired thickness (typically, about 5 to about 40 micrometers) is autodeposited on said metal substrate;
(b) rinsing said metal substrate having the layer of resin autodeposited therein with a chromium-free aqueous solution containing from about 0.1 to about 5 weight percent of calcium nitrate at a temperature of about 20xc2x0 C. to about 100xc2x0 C. for a time effective to improve the anticorrosive properties of the resin; and
(c) curing the layer of resin autodeposited on said metal substrate following rinsing step (b).
The process described herein does not require the use of chromium compounds of any type, yet furnishes coatings which effectively protect metallic substrates against corrosion.
Metal substrates which can be better protected against corrosion by application of the process of this invention comprise iron, tin, nickel, lead, chromium, zinc, aluminum, or alloys thereof, especially steel (e.g., cold rolled steel, galvanized steel), as well as surfaces which have been coated with one of said metals or its alloys.
The organic resins to be autodeposited on the surfaces of the metal substrates may include a variety of resin materials in emulsion (latex) or dispersion form as known from numerous publications. Resins based on epoxy resins such as glycidyl ethers of polyhydric phenols (e.g., bisphenol A) are particularly suitable for use in the present invention. The epoxy resin emulsions, in addition to one or more epoxy resins, may contain cross-linkers, curatives, emulsifiers, coalescing solvents, accelerator components, and the like. Such epoxy resin-based autodeposition coating systems are described, for example, in U.S. Pat. Nos. 4,233,197; 4,180,603; 4,289,826; and 5,500,460 and in international Publication No. WO 97/07163 (corresponding to U.S. Serial. No. 60/002,782, filed Aug. 16, 1995), the teachings of each of which are incorporated herein by reference in their entirety. It is believed that other suitable resins may include polyethylene, polyacrylates, styrene-butadiene copolymers, phenolic and novolac resins, urethanes, polyesters, vinyl chloride homo- and copolymers, vinylidene chloride homo- and copolymers and the like, although the alkaline earth metal compound, concentration and rinse temperature may have to be varied from what is described in the Examples section hereof in order for the corrosion resistance of the resulting coatings to be effectively improved.
For the actual coating procedure, the resin is autodeposited according to known methods on metal surfaces which preferably have been chemically and/or mechanically cleaned in the conventional manner. This type of process is described in U.S. Pat. Nos. 3,791,431; 4,186,219 and 4,414,350, all of which are incorporated herein by reference in their entirety, as well as in many other patents. If desired, the uncured coatings may be rinsed with water alone immediately after the actual coating step.
The alkaline earth metal compound used in the rinsing step must be soluble in water. Preferably, the alkaline earth metal portion of such compound is calcium. Preferably, the anion portion of such compound is nitrate. Calcium nitrate, for reasons which are not well understood, has been found to be especially effective in improving the corrosion resistance of autodeposited coatings. Illustrative examples of other suitable compounds include calcium chloride, calcium acetate, calcium formate, barium nitrate, barium acetate, and magnesium benzoate. Mixtures of alkaline earth metal compounds may be used. The alkaline earth compound need not be of high purity; technical or industrial grade materials can often be employed, provided the impurities present do not interfere with the development of the desired anticorrosion properties of the cured coating. For example, the calcium nitrate granules sold under the designation Norsk Hydro CN by Norsk Hydro, which contain about 80% calcium nitrate, 10% ammonium nitrate, 1% strontium nitrate and 15% water, have been found to be quite effective in the rinse process described herein when dissolved in water.
While not necessary to obtain significant improvement in corrosion resistance, other substances besides the alkaline earth metal compound(s) could be present in the aqueous rinse. A major advantage of the present invention is that there is no need to use chromium compounds in the rinse.
Although the concentration of the alkaline earth metal compound in the rinse solution is not believed to be particularly critical, an amount must be present which is sufficient to enhance the resistance of the resulting substrate towards corrosion. This minimum amount will vary depending upon the resin composition used, the alkaline earth metal compound selected, the rinse temperature, duration of rinsing, and the like, but may be readily determined through minimal experimentation. Typically, concentrations of from about 0.1 to about 5 percent by weight will suffice. Generally speaking, better corrosion resistance is obtained as the alkaline earth metal compound concentration in the rinse solution is increased. However, resistance to brake fluid and solvents and the appearance of the coating may be adversely affected at high alkaline earth metal compound levels.
The metal substrate autodeposition-coated with the uncured resin as described above is contacted with the rinse solution containing the alkaline earth metal compound according to known methods. For example, the metal substrates may be immersed or dipped in the rinse solution, spray-treated with the solution, roll-coated, or treated with a combined spray/dip procedure. Multiple rinses may be performed if so desired. The duration of treatment typically is from a few seconds to a few minutes, with a period of from about 30 seconds to about 5 minutes being preferred. During said treatment, the alkaline earth metal compound solution is generally maintained at a temperature of from about 20xc2x0 C. to about 100xc2x0 C. In at least certain embodiments of the invention, coating edge coverage is generally improved by increasing the rinse temperature from room temperature to about 50 degrees C. Typically, however, higher alkaline earth metal compound concentrations are needed at higher rinse temperatures.
Following the rinsing step, the coated metal substrates may be cured. Generally speaking, further rinsing with water alone is not desirable since such rinsing tends to degrade the improvements in corrosion resistance obtained by the alkaline earth metal compound rinse. Curing may be performed in any known manner, for example by heating (preferably baking) at an elevated temperature (e.g., about 50xc2x0 C. to about 300xc2x0 C.). The selection of the particular curing temperature will depend upon the type of resin, cross-linking agent, and coalescent used for the coating, among other factors.